Rigid suspension-bridge.



Patented Apr. 2, 1901. A. v. H. L. GISCLARD.

- RIGID SUSPENSION BRIDGE.

(Application filed July {1, 1900.}

2 Sheets-Sheet I.

l/Y/T/VISSSES l/YVIE/YTOR 2 W MM Mi Q M.

NITEED TATES ATENT FFICE.

RIGID SUSPENSION-BRIDGE.

SPECIFICATION formingpart Of Letters Patent No. 671,133, dated April 2, 1 901.

Application filed July 5, 1900.

To all whom, it may concern:

Be it known that I, ALBERT VICTOR HIPPo- LYTE LEON GISCLARD, engineer, a citizen of the Republic of France, residing in Vincennes, Seine, France, have invented new and useful Improvements in Rigid Suspension-Bridges, which invention is fully set forth in the following specification.

The new type of suspension-bridge which I will proceed to describe and which constitutes the object of the annexed application for patent is characterized by the special arrangements of its two' suspension-trusses. These trusses are provided with three hinges and appear in this respect to closely approach the types already known, such as the trusses of thesuspension-bridge at Pittsburg,in America. They differ, however, from many other points of view, for, as will be seen from the following explanation, they may, by reason of their special properties, be constructed in an entirely different manner and even dispense with cross-bracing. The principle upon which the construction is based can, first of all, be stated as follows: In each of the two half-trusses of which the suspension of the bridge is composed the upper and lower members are constituted by cables, chains, or hinged bars, the design of these members being so determined that they shall be in tension under all combinations of accidentaloverloads which may be exerted upon the roadway of the bridge. a

In the accompanying drawings, Figure 1 is a diagrammatic View of a suspension-bridge. Fig. 2 is a fragmentary side elevation of a suspension-bridge embodying one form of my present invention. Fig. 3 is a similar view showing another adaptation of my invention. Fig. 4 is a similar view showing another modification of my invention. Fig. 5 is an elevation of the type shown diagrammatically in Fig. 4.

In order to explain how this result may be obtained, let us first consider Fig. 1, in which CL and b designate the two points of suspension of the truss, a c d and b c e the two in variable half-trusses which compose it, and c the medianj oint which connects the two half-trusses. Suppose now that,withoutspecifyinganything beforehand as to the mode of constitution of these two half-trusses a c d and b o e, we as- Serial No. 22,474. \No model.)

sume, however, a pm'ort', the dead-weights of the different elements of the bridge to be given.

For any method of arrangement of the accidental loads which may be exerted on the floor of the bridge these loads, in combination with the existing deadweights, would give us total loads P P P, &c., which, with the unknown reactions f and g passing through the tops of the two towers, would represent the whole of the external forces which are exerted upon the structure of the bridge. Now of all the infinite number of polygons of forces which correspond to the known forces P P P 85.0., let us select and consider more particularly that which passes through the .three points a, c, and b. This polygon, which always exists and of which there is but one, since, in addition to a given system of forces,

three conditions suffice to determine a polygon of forces, is designated in statics by the name polygon or curve of forces. Granted this, nothing prevents us admitting that for each possible mode of arrangement of accidental loads upon the roadway of the bridge we might by known means construct the curve of forces corresponding to this system of accidental loads and to the dead-weights of the different elements of the bridge. We should thus obtain as'many difierent lines as there are admissible hypotheses of distribution of the loads that is to say, an infinity of curves of forces forming together a double set or pencil converging at a, b, and c. It is easy to determine, either by analysis or by a series of graphic diagramsthat is to say, by means of known methods of calculation, and for this reason it is not necessaryfor me to indicate it here-the form of the upper curve and lower curve which serve as exterior envelops for the collection of curves comprised in this set. Whatever it may be, let us admit that we adopt these envelops, or, more exactly, polygons, inscribed in these two curves for upper and lower contours of the truss. Hence I can by uniting these two polygons by a series of diagonal braces form two simple reticular systems suspended on both sides from the two fixed points a and b and connected together by the center hinge c, as shown in Fig. 2.

I complete the structure by suspensionrods attached, on the one hand, to the lower ties of the truss and, on the other hand, supporting at h 'zlj 70, &c., the cross-beams or transverse stays which support the longitudinal beams or girders of the floor of the bridge.

The following important proposition,which constitutes the characteristic feature of the new bridge, can be demonstrated by statics viz., that whatever may be the distribution of load put upon the floor of the bridge the dil ferent members or sect-ions which constitute the upper and lower chords of the truss are constantly in tension. It is likewise easy to show that the strains developed in the web memberssuch as Z m, m n, or n owill be compressions or tensions, according to circumstances-that is to say, according to the,

disposition and the relative intensities of the loads which are put upon the floor of the bridge; but this fact is of itself of no importance relatively to the conclusion which I will immediately draw-this is, that all the members which form the upper and lower contours of the two reticular systems being always in tension it results that these two half-trusses can never get out of their proper plane, and consequently it will be quite unnecessary to cross-stay them. l/Vhether they be in tension or compression, then, the Web members or braces Z m, 'm n, or n 0 may remain separate without being connected to the similar elements at the other side of the bridge, which will allow, notwithstanding the reduced height of the parts situated near the middle of the bridge, ofleaving an entirely free passage between the two trusses which support the roadway of the bridge. This in all its simplicity is the essential principle upon which the new type of structure is based. It will be understood, however, that I am not claiming here, broadly, astructure built upon such principle, except in so far as the structure which forms the subject of the claims at the end of the specification may be said to be built upon such principle. The object of Figs. 1 and2 and thedescription thereof is to make clear the elementary principle which I have used as astarting-point in developing the invention illustrated in the other figures of the drawings. From itthere may be de* duced a much more general type, of which the preceding is in some sort onlya particular case. Nothing in point of fact prevents us from supposing that the two upper hinges a, and binstead of being situated upon the tops of the two towers of the bridge may be brought closer together into the body of the truss, Fig. 3. Hence by prolonging the ideal curves of forces, of which we have already spoken, beyond these two points and up to the towers of the bridge two sets of divergent curves may be constructed, theenvelops of which will serve, as before, to determine the upper and lower contours of the two corresponding truss portions. It will suflice for completing the structure to connect these curves by web members or braces. It could, moreover, be shown without difficulty and by a method of reasoning entirely analogous to that which has already served that the elements which would constitute the external contours of these two new truss portions are always in tension. The form thus obtained is rigid and capable of free expansion, the same as in the preceding construction; but in place of a single point of attachment there are two such points at each extremity, which necessitates at each side and on each of the towers two expansion-carriages placed one below the other.

It is useful to here remark in passing that the properties which I have just enunciated would exist if instead of the enveloping curves of which I have spoken there were taken for contours of the truss curves exterior to'these sameenvelops. fore, that such exterior curves are within my invention. Thisarrangement, the eiiect of which would be to increase the height of each of: the-half-trusses, would consequently possess the advantage of: in a certain degree reducing the elastic deformations of the truss; but it would, on the'other hand, present the drawback of increasing the expense, whence it results that while-employing it according to circumstances it should never be resorted lJO-GXOBPi) in a just degree.

With regard to the mode of erecting the bridge, this can be deduced without difficulty from-the principle upon which its construction is based. Itis thus that in Figs. 2 and 3, which give the two principal types of the longitudinal profile of thetrusses, the upper and lower contours of=the reticular systems represent either chains formed-of hingedbars or metallic cables extended without break from one pillar to theother. The lineswhich form the interior elements of the bracing represent, on thecontrary, rigid bars capable of taking compression or extensionindil'ferently. These will therefore be composed of either metallic tubes or iron'or steel-sections employed separate from each other or connected together, so as to form hollow metal columns of the kind-employed in American hinged girders. These bars will be terminated at both sides by eyes into which fit the bolts or pins which form the tiesof the webbing. As to these,they will be retained on the cables or the chains which-form the truss members by pieces the form ofwhich w-illevidently depend upon the mode of' constitution of thesemembers.

The principle which I have established and which I have employed to determine the outline and-the mode of constitution of the upper and lower chord members of the truss can be extended to the other parts of the structurethat is, to the webbing. It is sufficient to generalize, as follows: Give to the entire structure a form such that the absolute value of the strain developed in each of its members. under the sole influence of the permanent load may be always greater than the It will be understood,- there strain of contrary direction due solely to the accidental or moving loads which may be exerted upon the roadway of the bridge. It is evident that if this condition is fulfilled the strains will never change in direction in any part of the truss-that is to say, the parts in tension will always remain in tension and the parts in compression will be always in compression. Fig. 4 represents diagrammatically a modification of my system designed according to this principle. In this figure the heavy lines represent the parts which are always in compression and the fine lines the parts always in tension. It will be seen how relatively few in number are the parts in compression. Fig. 5 represents the same modification as it appears when actually constructed.

In order to make the diagram of the bridge, I maintain the upper members on both sides in a straight line from the center hinge to the summits of the two towers; buta slight curve might without inconvenience be given to them either above or below these straight lines, pro vided always that one keeps sufficiently above the upper envelop of curves of force. As regards the lower members, I draw them as follows: 1 first consider the odd-numbered apexes-that is to say, the apexes l 3 5 7, &c., taken in the order or sequence of every other one, starting from the first which comes after the center hinge cand I place them upon the lower envelop of the curves of force or underneath this envelop. The elevation or depression of each even-numbered apex is de- 2, termined by the positions of the two odd apexes between which it is situated. It is easy, in fact, to ascertain that, the positions of the two apexes 3 and 5, for example, being determined, it suffices to progressively lower the apex 4 to find an upper limit of all the positions of this apex such that the diagonal braces 3 and 5 Fig. 4, shall be always in tension under all possible variations of excess load. The diagram thus obtained for the member will present a succession of salient and reentrant angles, the latter always falling upon the odd apex, with the exception of the first and last apexes, which will remain in any and every case slightly salient. It is evident, moreover, that for every given case the exact form of the profile of the truss will depend upon the conditions under which all the possible variations of accidental excess load will be exerted upon the floor of the bridge. The mode of construction of the truss will be in conformity with what has been already stated. The parts in tension will be formed of cables, flexible plates, or hingedbars, while the parts in compression will be composed of columns of suitable form.

Referring to Fig.5, the line C D F G H I K per chord. The lower chord is distinguished by a series of bends, which I believe to be a novel feature in structures of this sort. By reason of the condition that the odd-n umbered apexes (the central apex being numbered 0) are on the lower envelop of the curve of forces and that the even-numbered apexes are placed below the said envelop each member of the upper and lower chords is always-in tension whatever he the distribution of thelive load. The exact position of the even numbered apexes of the lower chord with respect to the coordinate axes ac and y through the center hinge is determined by the condition that the stress in each of the web members shall be always in the same directionthat is, always tension or always compression in a given member. By the use of an alternatively salient and reentrant form of the lower chord I make it possible with any load whatever to design the truss so that the permanent tension in the diagonal braces due to the dead load shall always exceed the accidental compression which would be due to the live load. The vertical braces or columns similarly are always in compression.

The last modification, Figs. 4c and 5, constitutes the most complete and most original application of the theoretical principle upon which my new system of truss is founded. It is very economical, and notwithstanding the large proportion of flexible tension members which enter into its composition it is susceptible of providing trusses of exceedingly great rigidity.

The invariability of the character of the stress as tension or compression in each of the members permits each member to be made in the form best designed to resist the particular stress to which it is to be submitted. By reason of the invariability of direction of the stresses at each apex I avoid the objection which is usually experienced due to the play of the pins in the eyes of the bars, and especially the characteristic rattling which is usually produced in pin-connected trusses under the action of rolling loads. The chord members and also the diagonal web members being always in tension, not only does each truss remain always in stable equilibrium in its own plane, but it exerts a considerable tendency to return to such plane against any force tending to move it out of such plane, thereby avoiding the necessity of transverse or wind bracing. On the whole, economy and rigidity are the most marked advantages of my improved truss. The modifications of Figs. 1 to 3 may be employed with advantage in the erection of certain constructive works, and in all cases they seem to possess, from the point of view of economy and lightness, an incontestable superiority over the other systems of triplehinge truss already known.

In recapitulation, I claim as my invention- 1. A three-hinged suspension-truss the contour of which is the envelop 0f the curves of force passing through the three hinges, and the end hinges of which are in the body of 5 the truss.

2. A three-hinged suspension-truss the contour of Which is the envelop of the curves of force passing through the three hinges and the lower chord of which presents alterna- 1o Lively salient and rentrant angles at the alternate apexes,a s and for the purpose described.

In testimony whereof I have signed this specification in the presence of two subscribing Witnesses.

ALBERT VICTOR HIPPOLY'IE LEON GISOLARD.

Witnesses:

J ULES ARMENGAUD, J eune,

J. ALLISON BOWEN. 

